In the vapor-liquid contact art, it is highly desirable to utilize methods and means that efficiently improve the quality as well as the quantity of the end products without increasing reflux rates or by the uneconomical use of introduced utilities, such as steam. Close fractionation and/or separation of the feed stock constituents and the elimination of harmful or undesirable residual elements, such as solids, conradson carbon and metals which are present in many chemical and petroleum feed stocks, as well as for purity are essential. Mass transfer, heat transfer, fluid vaporization and/or condensation, whereby one of the fluids can be cooled with a minimum pressure drop through and in a particular zone or zones of minimum dimensions defining the area and volume thereof, are additional prerequisites of efficient operation.
In the vapor-liquid contact art there are three basic fundamental process situations normally involved:
1. The superficial flow rate or mass of the vapor decreases as it ascends through a vapor-liquid contact vessel or a portion thereof;
2. The superficial flow rate or mass of the vapor increases as it ascends through a vapor-liquid contact vessel or a portion thereof; and
3. The vapor mass remains substantially constant without any significant fluctuation as it ascends through a vapor-liquid contact vessel or a portion thereof.
Illustrative practical cases of the three basic process situations are: (a) a vacuum tower in a petroleum refinery for situation 1; (b) a quench column or a desuperheater for situation 2; and (c) a fractionator operating under high vacuum for situation 3.
The methods and apparatus of the present invention find application in all three of the basic process situations listed above. In the detailed description which follows, the invention will be disclosed and discussed primarily in the context of the first situation, and its application to the other two situations will be briefly summarized for those skilled in the art.
For the many types of continuous separation processes incorporating the use of both concurrent and countercurrent vapor-liquid contact, it is desirable to utilize equipment that yields maximum through-put capacity and maximum vapor-liquid exchange efficiency while maintaining minimum pressure drop between the vapor feed stock inlet and the top overhead discharge of the vapor-liquid contact vessel.
Vapor-liquid contact efficiency is directly related to the superficial vapor energy, because the vapor energy creates intimate vapor-liquid contact by turbulence and/or mixing between the ascending vapor and the descending liquid through the height of a given contact zone. If the vapor energy is too low, the efficiency per foot of zone is low and a greater height of the zone, together with increased tower height, is required to achieve the separation or the desired function in said zone. This increases the capital cost of the equipment and furthermore can result in an undesirable increase in pressure drop through the height of the contact zone.
Vapor-liquid contact apparatus must have sufficient surficial and surface contact area to encourage intimate vapor-liquid contact without unduly restricting the flow of the ascending vapor or its countercurrent contact with the descending liquid. The greater the distance the ascending vapor must traverse in a particular contact apparatus of a given configuration, with its flow area obstructions to the flow of said ascending vapor, the greater the pressure drop through the apparatus will be for a given vapor rate.
For many services, and more particularly in vacuum service, very low pressure drop is desired. In addition, a high pressure drop through a given vapor-liquid contact apparatus reduces the capacity of the contact apparatus, since an increase in the vapor rate through-put with an accompanying and undesirable or excessive increase in pressure drop causes a hold-up of the descending liquid, and results in said apparatus flooding because it can no longer accept a desired increase in said vapor rate due to the vapor capacity having been reached and exceeded. The preceding also applies to descending liquid flow rates because a pressure drop point is reached which results in flooding of the contact apparatus since its liquid capacity has then been reached and exceeded.
Various means have been developed in this art in an effort to obtain greater capacity at the price of a sacrifice of efficiency, or greater efficiency at the price of a sacrifice of capacity. For those means known in the art which produce maximum capacity, not only is efficiency sacrificed or capital investment greatly enlarged by increased vessel height and/or diameter, but also the range of operation is materially narrowed between the capacity flood point and a minimum through-put rate that might be desired. This is a considerable disadvantage because many fractionating or vapor-liquid contact systems are required, by market or seasonal conditions, to operate at reduced rates well below the designed maximum operating rate.
In the vapor-liquid contact art, bulk packing, such as Raschig rings and saddles, has been used to obtain desirable efficiency values resulting from surficial obstructions and tortuous vapor paths for the ascending feed stock vapor. Bulk packing with its lower capacity requires a larger diameter tower to obtain the maximum through-put capacity commensurate with good separation efficiency. Random bulk packing in the bed height and diameter normally required is subject to and encourages poor ascending vapor distribution or descending liquid distribution, resulting in the channelling and/or bypassing of ascending vapor and/or descending liquid with little or not contact. Bulk packing positioned in place in a preselected pattern magnifies the problem of poor vapor-liquid distribution as well as producing a loss in efficiency due to the lack of turbulence by vapor-liquid passage streamlining. Furthermore, in certain types of service such maldistribution of the vapor and liquid causes coking or plugging of the bulk packing because of areas of quiescence and/or lack of turbulence.
It is noted that the prior art, namely, Winn and Winn et al. U.S. Pat. Nos. 3,079,134 and 3,343,821, discloses vapor-liquid contact apparatuses having large vapor and liquid passage areas to ensure proper comingling and contact of ascending vapors with descending liquid substantially throughout the entire volume occupied by the grid beds. In addition, these disclosures provide grids of substantial structural strength to minimize the use of auxiliary supports in vapor-liquid contact towers and said grids, with their excellent vapor-liquid mixing characteristics and controlled turbulence of vapor and liquid, greatly increase the through-put capacity and thereby permit the use of smaller towers with lower pressure drop than can be accomplished with random bulk packing material.
The vapor-liquid contact grids of these disclosures share, to some extent, a characteristic common to contact grid generally. At low vapor rates, their efficiency drops off, which limits the practical minimum through-put rate to a relatively high value.